About Us

The study of public-supply well vulnerability to contamination from compounds
commonly found in the environment is one of five national priority topics being
addressed by the National Water-Quality Assessment (NAWQA) Program. The
Transport of Anthropogenic and Natural Contaminants to Supply Wells (TANC) study
began in 2001 with the following general objectives...

Identify the dominant contaminants and sources of those contaminants in
public-supply wells in representative water-supply aquifers across the
Nation

Assess the effects of natural processes (such as degradation) and human
activities (such as irrigation) on the occurrence of contaminants in
public-supply wells in representative aquifers

Identify the factors that are most important to incorporate into
public-supply well vulnerability assessments in different settings and at
different spatial scales

Develop simple methods and models for screening public-supply wells for
vulnerability to contamination in unstudied areas and from newly emerging
contaminants

Increase understanding of the potential effects of water-resource
development and management decisions on the quality of water from public-supply
wells

Unique Characteristics of the TANC Study

Sampling at different depths -- The screened or open intervals of
public-supply wells are commonly from tens to hundreds of feet in length;
therefore, water from these wells is generally a mixture of waters of different
ages that enter the well at different depths and are associated with different
potential sources of contamination. Using a USGS-developed sampler (Izbicki
and others, 1999 and
Izbicki,
2004), the TANC study is collecting samples at multiple depths in pumping
public-supply wells to ascertain where and how contaminants from different
sources enter the wells.

Evaluating multiple settings and scales -- Consistent methods are
being used to collect and analyze data, and investigations are being
conducted at both regional (tens to thousands of square miles) and local
scales (less than 10 square miles). Regional-scale investigations are
allowing for comparison of important contaminant-transport mechanisms in
water-supply aquifers across the Nation. Local-scale investigations build
upon the regional-scale efforts and are focusing on understanding common
transport and transformation processes within the contributing areas of
public-supply wells.

Exploring the consequences of uncertainty -- To make
informed decisions about activities at a particular location,
decision-makers need to know whether the location is
contributing recharge to public-supply wells. They also need
information about traveltimes between potential contaminant
sources and public-supply wells. Because this information cannot
be measured directly, decision-makers must rely on estimates
that are inherently uncertain (due to limitations in the
methods). The TANC study is exploring the consequences of this
uncertainty, and helping decision-makers understand these
consequences, by comparing estimates from traditional and
probabilistic modeling approaches with actual water-quality data
from public-supply wells.